Temperature-resolved second harmonic generation (TR-SHG) and SHG
microscopy were used to study under normal pressure the solid–solid
transition mechanism occurring between the two monotropically related
polymorphic forms (metastable Pna21 and
stable P21/n) of 3-hydroxybenzoic
acid (MHBA). The activation energy E
a (as
a measure of the barrier energy) of the irreversible transition was
determined via isothermal TR-SHG (137–144 kJ·mol–1). It fits well with that determined from differential scanning calorimetry
(139 kJ·mol–1). Regarding the two crystal structures,
optical microscopy observations, and kinetics parameters from TR-SHG,
a destructive/reconstructive mechanism is proposed for the solid–solid
transition. The present study clearly demonstrates that TR-SHG is
a relevant and accurate technique for monitoring solid–solid
phase transitions.
The application of powder second harmonic generation (P-SHG), temperatureresolved SHG (TR-SHG), and SHG microscopy (SHGM) in the characterization of bulk crystalline samples is illustrated. P-SHG applied to powder samples can be an extremely sensitive approach to detect the absence of an inversion center in crystalline structures, TR-SHG serves to monitor temperature-induced phase transitions, and SHGM is used in the detection of non-centrosymmetric zones inside a heterogeneous material. These methods are of great relevance, e.g., in the pharmaceutical industry where crystalline active pharmaceutical ingredients are often made of a single enantiomer and are therefore non-centrosymmetric. Herein, several examples are provided to describe how a given SHG signal should be interpreted. A general procedure to carry out a P-SHG experiment is illustrated in detail.
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